Opencl atomic_add() function returns wrong value

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OpenCL

Have a good time

i have a problem in OpenCL 1.2.
Look, i have an array as global in the kernel and the group size is 1000.
The problem is that the atomic_add() function doesn't work correctly.

My kernel code is :

buffer[3] = 100;
atomic_add(&buffer[3], 1);

if i create 1000 threads, i expect the value of buffer[3] will be 1100, am i right?
but the behavior of the program is undefined.
sometime it will be 1100, sometimes 1064, sometimes 1093 and ...

What I have tried:

i also enable the opencl extension like below :

#pragma OPENCL EXTENSION cl_khr_global_int32_base_atomics : enable

but the problem is still exists.
in the another project i'de create a simple opencl project and the atomic_add works currectly, and i've checked almost the entire of project configuraton but i don't know the problem where is.

can you help me?
thanks